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Psychoactive Drugs

Drugs that affect states of con­scious­ness are called psychoactive drugs. A discussion of addiction is found later in Chapter 14 (Frontiers of Psychology).

All psychoactive drugs have certain things in common.

1. They are able to cross the blood/brain barrier. In order to affect consciousness, a drug must penetrate the biological filter that prevents many substances from reaching the brain.

2. They alter brain chemistry at the level of individual brain cells. Most drugs act at the level of neurotrans­mitters. Drugs block transmitter re-uptake, or interfere with transmitter synthesis, or mimic the effects of transmitters.

What characteristics do all psychoactive drugs have in common?

3. Effects of a drug depend on dosage. Drug effects may vary from none at all (at very low dosages) to toxicity or poisoning (at high levels). A dose/response curve is a graph relating dosage to some measure of a drug's effect.

What is a dose/response curve?

4. Drug effects are altered by prior experience with the drug. Drugs can sensitize individuals to future exposures, leading to a larger reaction when the same drug is taken again.

Continual exposure to a drug leads to the opposite phenomenon: tolerance. The body becomes accustomed to the drug. Few drugs produce the same effects in experienced users as they do in beginners.

5. Their effects depend on expectancy. In all psychoactive drugs yet studied, the response to the drug depends partly on what people expect to happen.

Individuals who think they are receiving alcohol will act drunk, even if they receive drinks without alcohol. Similar things happen with marijuana and other drugs.

6. They can be addictive. If a psychoactive drug produces pleasurable effects, some people will chose to repeat the experience. As they repeat the experience, their bodies adapt to the presence of the drug, sometimes producing uncom­fortable or dangerous withdrawal reactions when they try to quit.

Psychologists studying addictive behavior find the failure rate for overcoming all addictions is about the same, whether the addiction is alcohol, cigarettes, or heroin.

Rating the Dangers of Drugs

One way to operationally define the danger of a drug is to calculate a dose/toxicity ratio. This ratio is formed by dividing the minimal effective dose of a drug by the amount that will kill a person. This provides a measure of a drug's dangerousness.

What is the dose/toxicity ratio?

Robert Gable of Claremont Graduate University did extensive research on dose/toxicity ratios. He wrote on his blog:

It started about 25 years ago when my college-age son came into my study and asked "MDMA is like alcohol, isn't it?"

I told him that I had read an article in Science that MDMA (Ecstasy) "screws up rats' brains"... I promised my son that I would get an answer to his question. I have been working on the problem recreational drug toxicity ever since. What has moti­vated me? Fear, frustration, and hope.

A pdf of Gable's conclusions (Gable, 2006) can be downloaded here. Gable found that GHB (Gamma Hydroxy­butyrate, a date-rape drug) and heroin "have a lethal dose less than 10 times their typical effective dose" while cocaine, MDMA (ecstasy), and alcohol "all have a lethal dose 10 to 20 times the effective dose." He continued:

A less toxic group of sub­stances, requiring 20 to 80 times the effective dose to cause death, include Rohypnol (flunitrazepam or "roofies") and mescaline (peyote cactus). The least physiologically toxic sub­stances, those requiring 100 to 1,000 times the effective dose to cause death, include psilocybin mushrooms and marijuana, when ingested.

I've found no published cases in the English language that document deaths from smoked marijuana, so the actual lethal dose is a mystery. My surmise is that smoking mari­juana is more risky than eating it but still safer than getting drunk.

Alcohol thus ranks at the dangerous end of the toxicity spectrum. So despite the fact that about 75 percent of all adults in the United States enjoy an occasional drink, it must be remembered that alcohol is quite toxic.

Indeed, if alcohol were a newly formulated beverage, its high toxicity and addiction potential would surely prevent it from being marketed as a food or drug. (Gable, 2006)

Gable also described another opera­tional definition of danger­ousness in recreational drugs. This is the capture ratio: the number of people who try a drug divided by the number who become regular or habitual users.

What is capture ratio? Which drugs are most dangerous by this measure?

Gable notes that "heroin and methamph­etamine are most dangerous by this measure," followed by cocaine, alcohol, nicotine, then caffeine and marijuana. Hallucinogens are least dangerous by this measure, because people do not become dependent upon them.

Other sources are harder on tobacco. The U.S. Co-morbidity Study estimated the following capture ratios: Tobacco 33%, Heroin 23%, Cocaine 17%, Alcohol 15% (Iversen, Iversen, Bloom, and Roth, 2008).

Different Categories of Drugs

Depressants are drugs that lower the overall level of activity in the nervous system. Among the chemically distinct families of depressant drugs are sedatives, hypnotics, and alcohol. All are capable of producing relaxation or sluggish and lethargic behavior. In large doses, all can produce coma or death.

Sedatives include drugs like PCP (phencyclidine), a drug commonly abused in the 1970s and 1980s, and barbiturates, used in sleeping potions and pills from the 1800s through the first half of the 20th Century. Depressants are often potentiated (made much more powerful) by combination with alcohol.

What are depressants?

Hypnotics include muscle-relaxants like methaqualone (quaaludes). Like PCP, its manufacture requires sophisticated processes. Both were taken off the streets by limiting the number of factories authorized to produce their constituent chemicals.

What are effects of stimulants?

Stimulants are drugs that increase overall activity and excitability in the nervous system. Commonly known stimulants are caffeine, amphetamines ("speed"), and cocaine.

Stimulants increase alertness and activity. In large amounts, they cause nervousness, shaking movements or jitters, insomnia, and irregular heart­beats. Sometimes they cause anxiety or panic states accompanied by hyper­ventilation and lightheadedness.

Opiates are drugs derived from the opium poppy. The word opioid was once used only for synthetic drugs similar to natural opiates in chemical structure and action, but recently the term has been used to label all opiates, natural and synthetic.

Opiates include heroin, morphine, and codeine. Synthetic opioids include Demerol, Fentanyl, Dilaudid, and Methadone. Endorphins are natural opioids, pain-killing substances secreted within the body.

To pharmacologists, opiates are the only drugs properly termed narcotics. Cocaine, marijuana, sedatives, and LSD are not narcotics, unless the term is used to refer to all illegal drugs.

Marijuana (cannabis) is in a category by itself, because it does not resemble other drugs either in chemical structure or effect. The active ingredient in marijuana is delta-4-tetrahydrocannabinol or THC.

Marijuana achieves its effect by altering levels of a transmitter, anandamide, that is not affected by other drugs. Ananda­mide receptors are very common in the human nervous system, so the transmitter must be involved in important brain processes, but its normal function is not yet clear. Hashish is a form of concen­trated cannabis resin.

Why is marijuana in a category by itself?

Hallucinogens or psychedelic drugs include LSD (lysergic acid diethylamide or "acid"), mescaline (the active ingredient in peyote buttons), and psilocybin (the active ingredient in psychoactive mushrooms).

Hallucinogens cause a dreamlike state ("trip") with dramatic alterations of thought and emotion. The major psychedelics are all chemically related. Their hallucinogenic effects are thought to be caused by changes in levels of the neurotransmitter serotonin.

The internet provides many web sites about drugs and their effects, ranging from sites focusing on prevention of substance abuse (such as drugnet) to sites where users share experiences and cautions about drugs ranging from caffeine to hallucinogens (erowid).


Gable, R. S. (2006) The toxicity of recreational drugs. American Scientists, 94, 206-208.

Iversen, L., Iversen, S., Bloom, F. E., & Roth, R. H. (2008) Introduction to Neuropharmacology. Oxford: Oxford University Press.

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